食品专业英语 LESSON 4 Amino Acids And Proteins
Proteins are molecules of great size complexity and diversity They are the source of dietary amino acids both essential and nonessential that are used for growth maintenance and the general wellbeing of man These macromolecules characterized by their nitrogen contents are involved in many vital processes intricately associated with all living matter In mammals and many internal organs are largely composed of proteins Mineral matter of bone is held together by collagenous protein Skin the protective covering of the body often accounts for about 10 of the total body protein
Some protein function as biocatalysts (enzymes and hormones) to regulate chemical reactions within the body Fundamental life process such as growth digestion and metabolism excretion conversion of chemical energy into mechanical work etc are controlled by enzymes and hormones Blood plasma proteins and hemoglobin regulate the osmotic pressure and PH of certain body fluids Proteins are necessary for immunology reactions Antibodies modified plasma globulin proteins defend against the invasion of foreign substances of microorganisms that can cause various diseases food allergies result when certain ingested proteins cause an apparent modification in the defense mechanism This leads to a variety of painful and occasionally drastic conditions in certain individuals
Food shortages exist in many areas of the world and they are likely to
become more acute and widespread as the world’s population increases providing
adequate supplies of protein poses a much greater problem than providing
adequate supplies of either carbohydrate or fat Proteins not only are more
costly to produce than fats or carbohydrates but the daily protein requirement
per kilogram of bodyweight remains constant throughout adult life whereas the
requirements for fats and carbohydrates generally decrease with age
As briefly described above proteins have diverse biological functions structures and properties Many proteins are susceptible to alteration by a number of rather subtle changes in the immediate environment Maximum knowledge of the composition structure and chemical properties of the raw materials especially proteins is required if contemporary and future processing of foods is to best meet the needs of mankind A considerable amount of information is already available although much of it has been collected by biochemists using a specific food component as a model system
Amino Acids
Amino acids are the building blocks of proteins Therefore to understand the properties of proteins a discussion of the structures and properties o f amino acids is required Amino acids are chemical compounds which contain both basic amino groups and acidic carboxyl groups Amino acids found in proteins have both the amino and carboxyl groups on the acarbon atom aamino acids have the following general structure
At neutral pH values in aqueous solutions both the amino and the carboxyl groups are ionized The carboxyl group loses a proton and obtains a negative charge while the amino group gains a proton and hence acquires a positive charge As a consequence amino acids possess dipolar characteristics The dipolar or zwitterions form of amino acids has the following general structure
Several properties of amino acids provide evidence for this structure they are more soluble in water than in less polar solvents when present in crystalline form they melt or decompose at relatively high temperatures (generally above 200) and they exhibit large dipole moments and large dielectric constants in neural aqueous solutions
The R groups or side chains of amino acids and proteins these side chains may be classified in to four groups
Amino acids with polaruncharged (hydrophilic) r groups can hydrogenbond with water and are generally soluble in aqueous solutions The hydroxyls of serine heroine and tyrosine the sulfhydryl of thinly of cysteine and the amides of asparagines and glutamine are the functional moieties present in r groups of the class of amino acids Two of these the toil of cysteine and the hydroxyl of tyrosine are slightly ionized at PG 7 and can lose a proton much more readily than others in this class The amides of asparagines and glutamine are readily hydrolyzed by acid or base to aspartic and glutamic acids respectively
Amino acids with nonpolar (hydrophobic) r groups are less soluble in aqueous solvents than amino acids with polar uncharged r groups Five amino acids with hydrocarbon side chains decrease in polarity as the length of the side chain is increased The unique structure of praline (and its hydoxylated derivative hydroxyproline) causes this amino acid to play a unique role in protein structure
The amino acids with positively charged (basic) r groups at ph 67 are lysine argiine has a positively charged quanidino group At ph 70 10 of the imidazole groups of histidine molecules are prorogated but more than 50 carry positive at ph 60
The dicarboxylic amino acids asparic glutamic possess net negative charges n the neutral ph range An important artificial mealflavoring food additive is the monosodium salt of glutamic acid
Peptides
When the amino group of one amino acid reacts with the carboxyl group of another amino acid a peptide bond is formed and a molecule of water is released This can bond joins amino acids together to form proteins
The peptide bond is slightly shorter than otter single cn bonds This indicates that the peptide bond has some characteristics of a double bond because of resonance stabilization with the carbony1 oxygen Thus group adjacent to the peptide bond cannot rotate freely this rigidity of the peptide bond holds
the six atoms in a single plane the amino (_NH_) group does not ionize between ph o and 14 due to the doublebond properties of the peptide bond In addition r groups on amino acid residues because of starch hindrance force oxygen and hydrogen of the peptide bond to exist on a trans configuration Therefore the backbone of peptides and proteins has free rotation in two of the three bonds between amino acids
If a few amino acids are joined together by peptide bonds the compound is called a most natural peptides are formed by the partial hydrolytic of proteins however a few peptides are important metabolites Ansetime and carnosine are two derivatives of histamine that are found in muscles pf animals The biochemical function of these peptides is not understood
Glutathione occurs in mammalian blood yeast and especially in tissues of rapidly dividing cells It is thought to function in oxidative metabolism and detoxification
Duirng oxidation two moletcules of glutathiune join vin a disulfide bridge (SS) between two cysteine is not found in proteins
Other peptides functino as antibodies and hormones Oxytocin and hormones Oxytocin and vasopressin are examples of peptide hormones
Protein structure
Proteins perform a wide variety of biological functions and since they are composed of hundreds of amino acids their structures are much mere complex than those of peptides
Enzymes are globular proteins produced in living matter for the special purpose of catalyzing vital chemical reactions that otherwise do not occur under physiological conditions Hemoglobin and myoglobin are hemocontaining proteins that transport oxygen and carbon dioxide in the blood and muscles The major muscle proteins actin and myosin convert chemical energy to mechanical work while proteins in tendons (collagen and elastim) bind muscles to bones skin hairy fingernails and toenails are pertinacious protective substance The food scientist is concerned about proteins in foods since knowledge of protein structure and behavior allows him to more ably manipulate foods for the benefit mankind
Nearly an infinite number of proteins could be synthesized from the 21natural occurring amino acids However it has been estimated that only about 2000 different proteins exist in nature The number is greater than this if one considers the slight variations found in proteins from different species
The linear sequence of amino acids in protein is referred toast primary structure In a few proteins the primary structure has been determined and one protein (ribonuclease) has been synthesized in the laboratory It is the unique sequence of amino acids that imparts many of the fundamental properties to different protein and tertiary structures If the protein contains a considerable number of amino acids with hydrophobic groups its solubility in aqueous solvents is probable less than that of proteins containing amino acids with many hydrophilic groups
If the primary structure of the protein were not folded protein molecules would be excessively long and thin A protein having a molecular weight of 13000 would be 448 a thick This structure allows excessive interaction with other substances and it is not found in nature The threedimensional manner in which relatively close members of the protein chain are arranged is referred to as secondary structure
examples or secondary structure are the ahelix of wool the pleatedsheet configuration of silk and the collagen helix
The native structure of a protein is that structure which possesses the lowest feasible free energy Therefore the structure of a protein is not random but somewhat ordered when the restrictions of the peptide bond are superimposed on a polyamino acid chain of a globular protein a right handed coil the ∝helix appears to be one of the most ordered and stable structures feasible
the ∝helix contains 36 amino acid residues per turn lof the protein backbone with the r groups of the amino acids extending outward from the axis of the helical structure hydrogen bonding can occur between the nitrogen of one peptide bond and the oxygen of another peptide bond four residues along the protein chain the hydrogen bonds are nearly parallel to the axis of the helix lending strength to the helical structure since this arrangement allows each peptide bond to form a hydrogen bond the stability of the structure greatly enhanced The coil of the helix is sufficiently compact and stables that even substances with strong tendencies to participate in hydrogen bonding such as water cannot enter the core
A secondary saturation found in many fibrous proteins is the βpleated sheet configuration In this configuration the peptide backbone forms a zigzag pattern with the r groups of the amino acids extending alive and below the peptide chain Since all peptide bonds are available for hydrogen bonding this configuration allows maximum crosslinking between adjacent polypeptide chains and thus good stability Both parallelpleated sheet where the polypeptide chains run in opposite directions are possible Where groups are bulky or have little charges the interactions of the r groups do not allow the pleatedsheet configuration to exist silk and insect fibers are the best examples of theβsheet although feathers of birds contain a complicated form of these configuration
Another type of secondary structure of fibrous proteins is the collagen helix collagen is the most abundant protein in higher vertebrates accounting for onethird of the total body protein collagen resists stretching is the major component of tendons and contains onethird glycine and onefourth proline or hydroxyprolinethe rigid r groups and the lack of hydrogen bonding by peptide linkages involving proline and hydroxyproline prevents formation of an ∝helical structure and forces the collagen polypeptide chain into an odd kinkedtype helix Peptide bonds composed of glycine form interchain hydrogen bonds with two other collagen polypeptide chains and this results in a stable triple helix This triplehelical structure is called tropocollagen and it has a molecular weight of 3000000 Daltons
The manner in which large portions of it protein chain are arranged is referred to as tertiary structure This involves folding of regular unts of the secondary structure as well as the structuring of areas of the peptide chain that are devoid of secondary structure for example some proteins contain areas where ∝helical structure exists and other areas where this structure cannot form depending on the amino acid sequence the length of the ∝helical portions are held together by hydrogen bonds formed between r groups by salt linkages by hydrophobic interactions and by covalent disulfide(ss0 linkages
The structures discussed so far have involved only a single peptide chain The structure formed when individual (subunit) polypeptide chains interact to form a native protein molecule is referred to as quaternary structure The bonding mechanisms that hold protein chains together are generally the same as those involved in tertiary structure with the possible exception that disulfide bonds do not assist in maitaining the quaternary structures of proteins
第四课 氨基酸蛋白质
蛋白质错综复杂种样分子物质食物必须氨基酸非必须氨基酸源体利氨基酸满足生长发育修复组织维持正常健康生活求分子含氮特征参许种生命物质复杂联系生命程包括类哺乳动物中蛋白质起着机体改造成分作肌肉许体器官蛋白质构成骨骼中矿物质胶原蛋白保持起机体保护层—皮肤中蛋白质通常占机体蛋白质总量10左右
蛋白质生物催化剂（酶激素）作调节体化学反应基生命程生长消化代谢排泄化学转变成机械功等等受酶激素控制某体液渗透压pH值受制血浆蛋白血红蛋白蛋白质免疫反应必少抗体（改性血浆球蛋白引起疾病外杂质微生物入侵某摄入蛋白质防御机制产生明显变化时便发生体生物敏导致某体身出现种样疾病时急剧病情
食物短缺现象世界许区存着口增加问题愈愈尖锐愈普遍蛋白质供应足问题远碳水化合物脂肪供应足更严重蛋白质仅产出费碳水化合物脂肪产出费高千克天需蛋白质量造整成年期恒定天需脂肪碳水化合物量般着年龄增长逐渐减少
正面简述样蛋白质种结构性质生理功许蛋白质容易受周围环境系列微妙变化影响发生变化想现食品加工理想满足类需必须彻底解原料特蛋白质组成结构化学性质目前已方面量资料供利中部分生物化学家利某特定食物成分作模拟物系加收集
氨基酸
氨基酸蛋白质结构单元解蛋白质性质旧需讨氨基酸结构性质氨基酸含氨基含酸性羧基化合物蛋白质中氨基酸α碳原子时氨基羧基α氨基酸具般结构：
中性pH水溶液中氨基羧基呈离子状态羧基失质子带负电荷时氨基质子带正电荷结果氨基酸便具偶极特性氨基酸种偶极形式（两性形式）般结构：
氨基酸种性质反映种结构性质：易溶水易溶极性溶剂：晶体形式存时较高温度（般200℃）熔化分解中性溶液种显示出偶极矩介电常数
氨基酸侧链R基团氨基酸蛋白质化学性质产生重影响侧链分四类
带极性非荷电（亲水）R基团氨基酸水形成氢键通常溶水溶液丝氨酸苏氨酸酪氨酸羟基半胱氨酸硫氢基（硫醇）天冬酰胺谷氨酰胺酰胺基时出现类氨基酸R基团中功部分中半胱氨酸硫羟基酪氨酸羟基pH7时轻度离子化类中氨基酸更容易失质子天冬酰胺谷氨酰胺酰胺基容易酸碱水解分形成天冬氨酸谷氨酸
带非极性（疏水）R集团氨基酸水溶液中溶解性带极性非荷电R基团氨基酸带烃侧链五种氨基酸侧链侧链长度增加降低脯氨酸（烃基衍生物羟脯氨酸）独特结构种氨基酸蛋白质结构中独特位
pH6～7时带正电荷（碱性）R基团氨基酸赖氨酸精氨酸组氨酸赖氨酸带正电原氨基精氨酸具带正电荷胍基pH7时组氨酸分子中咪唑基10质子化pH6时50带正电荷
二羟基氨基酸（天冬氨酸谷氨酸）中性pH范围带净负电荷谷氨酸钠盐种重膳食调味造食品添加剂
肽
氨基酸分子氨基氨基酸分子羧基起反应时便形成肽键时释放出分子水种CN键众氨基酸连接起形成蛋白质
种肽键简单CN键略短说明肽键羰基氧振稳定作肽键具定双键特性样紧邻肽键基团转动肽键种刚性六原子保持面 H
肽键双键性质亚氨基（NH)pH0～14间均离子化外立体位阻现象氨基酸残基R基团迫肽键氧原子氢原子反式构型存肽蛋白质链氨基酸间三键中两键作转动
果少数氨基酸肽键连接起样化合物称肽数天然肽蛋白质部分水解形成少数肽重代谢产物鹅肌肽肌肽动物肌肉中组氨酸两种衍生物肽生物化学功目前清楚
谷胱甘肽存哺乳动物血液酵母中特快速分解细胞质组织中般认种肽具参氧化代谢解毒作功氧化程中两分子谷胱甘肽通两半胱氨酸残基间二硫键SS连接起蛋白质中未发现谷氨酸γ羰基半胱氨酸连成肽键
外具抗体激素功肽催产素抗利尿素肽激素例子
蛋白质结构
蛋白质执行种样生物功数百氨基酸组成结构远肽复杂
酶生物中产生球状蛋白质目专门催化某生物化学反应然话化学反应生理条件会发生血红蛋白肌红蛋白输送血液肌肉中氧二氧化碳含血红素蛋白质重肌肉蛋白—肌动蛋白肌球蛋白化学转变成机械腱中蛋白质（胶原蛋白弹性蛋白）肌肉粘连骨骼皮肤毛发指（趾）甲蛋白质类保护物质食品科学家关注食物蛋白质掌握蛋白质结构功方面知识更加工处理食品造福类
21种天然存氨基酸合成数蛋白质估计然界存约2000种蛋白质果考虑物种蛋白质存微差异蛋白质数目超数
蛋白质分子氨基酸直线排列次序作蛋白质级结构少数蛋白质级结构已确定已实验室里合成中种蛋白质（核糖核酸）正种氨基酸独特排列序赋予蛋白质许基特性程度决定二级三级结构果蛋白质中含量带疏水基团氨基酸水溶剂中溶解性带许亲水基团氨基酸蛋白质差
果蛋白质级结构折叠蛋白质分子会长细分子量13000蛋白质分子应448纳米长37纳米粗种结构物质发生度相互反应然样结构然界未发现蛋白质链中相互链节链节间三维排列方式蛋白质二级结构二级结构具体例子羊毛蛋白α螺旋结构蚕丝蛋白折叠片结构胶原蛋白螺旋结构
蛋白质然结构含低结构蛋白质结构意定规球状蛋白质聚氨基酸链受肽键约束时右螺旋（α螺旋）规稳定合理结构
圈α螺旋蛋白质链含36氨基酸残基氨基酸R基团螺旋结构轴线外伸出肽键氮够蛋白质链相距四氨基酸残基处氧形成氢键氢键差α螺旋轴线行赋予螺旋结构强度样排列肽键形成氢键加强结构稳定性螺旋圈非常紧密坚固象水样强烈参形成氢键趋势物质进入螺旋中央部分
出现许纤维状蛋白质中二级结构β折叠片结构构型中肽链呈锯齿形氨基酸R基团肽链方方伸展肽键供氢键形成种构型够相邻肽链间充分形成交联具良稳定性两种折叠片相邻肽链走相行折叠片走相反反行折叠片均存果肽链中R基团带种电荷R基团间相互作β折叠片形成蚕丝昆虫纤维蛋白β折叠结构例子鸟类羽毛中含种构型复杂形式
纤维蛋白种二极结构胶原螺旋胶原螺旋高等脊椎动物中丰富种蛋白质占动物体蛋白总量13胶原蛋白中含1＼3甘氨酸1＼4脯氨酸羟脯氨酸抵抗拉伸腱组分R基团刚性脯氨酸羟脯氨酸参肽式键合形成氢键原α螺旋结构法形成迫胶原肽链变成种零散结节式螺旋体胶原肽链中甘氨酸构成肽键两条肽链形成键间氢键产生种稳定三股螺旋三股螺旋结构称原胶原分子量30万道尔顿
蛋白质链中链段排列方式称蛋白质三级结构包括二级结构常规单元折叠二级结构肽链干区域结构化例某蛋白质中包含α螺旋结构存区域外形成种结构区域根氨基酸序α螺旋段长度赋予独特三级结构折叠部分R基团间形成氢键盐键疏水相互作价二硫键（SS）结合起
讨结构仅涉单肽链结构（亚单位）肽链相互作变成天然蛋白质分子时形成结构蛋白质四级结构蛋白质键结合起键合机制通常三级结构中述相例外情况双硫键参蛋白质四级结构保持
专业英语词汇
intricate a复杂错综缠结难懂
collagenous a胶原
globulin 球蛋白
plasma 血浆原生质
immunological 免疫
hemoglobin 血红蛋白
basic amino 碱性氨基
acidic carboxyl 酸性羧基
aqueous ①水 ②含水 ③水成
proton 质子氕核
dipolar 偶极两极
zwitterion 两性离子
crystalline ①结晶晶状 ②清澈
hydrophilic 亲水
serine 丝氨酸羟基丙氨酸
threonine 羟基丁氨酸苏氨酸
tyrosine 酪氨酸 3羟苯基丙氨酸
sulfhydryl 氢硫 ～enzyme 硫化氢解酶 ～ group 巯基
cysteine 半胱氨酸巯基丙氨酸
cystine 胱氨酸双巯丙氨酸
amide ①酰胺 ②氨化物
asparagine 天门冬酰胺
glutamine 谷氨酰胺
aspartic acid 天门冬氨酸丁氨二酸
glutamic acid 谷氨酸
proline 脯氨酸氮戊环[2]基羧酸
lysine 赖氨酸
arginine 精氨酸
histidine 组氨酸咪唑丙氨酸
quanidino 胍
imidazol n 咪唑13二氮杂茂
resonance n ①回声反响 ②[物]振鸣谐振 ③[医]叩响
imino 亚氨
steric a空间位
anserine 鹅肌肽
carnosine 肌肽
glutathione 谷胱甘肽
peptide肽缩氨酸
oxytocin n(垂体)叶催产素
vasopressin n叶加(血)压素加压素
carbonyl 羰基碳酰
hemoglobin 血红蛋白 hemo表示血
myoglobin 肌红蛋白
actin 肌动蛋白
myosin 肌球蛋白
tendon 腱筋根
collagen 胶原胶原蛋白
elastin 弹性蛋白
ribonuclease 核糖核酸酶
hydrophobic 疏水
restriction n 限制限定约束
vertebrate n脊椎动物 a①椎骨脊椎脊椎动物 ②(作品等)结构严密
kink n①(绳索头发)细结绞缠 ②奇想怪念头乖僻 ③(奇特)妙法 ④(颈背等处)
肌肉痉挛抽筋 ⑤[美](结构设计等)缺陷 vt纽结绞缠 vi纽结结
glycine 甘氨酸氨基醋酸
tropocollagen 原胶原
dalton 道尔顿
devoid a 缺乏没(of)
covalent 价 ～ bond价键
quaternary a①四组四部组成第四 ②四元四价季 ③[]第四纪
n①四四组第四组中组成部分 ②[数]四进制 ③[]第四纪
zigzag ①字形Z字形锯齿形②字形线条（道路壕沟装饰等）③蜿蜒曲折盘旋弯曲
专业英语总结
English Knowledge point
1 be of +名词相形容词
F： Proteins are molecules of great size complexity and diversity proteins are molecules of great size complexity and diversity roteins are molecules of great size complexity and diversity proteins are molecules of great size complexity and diversity
2 at PH 7 at neutral PH (介词at)
SOME GOOD SENTENCE
1 Amino acids are the building blocks of proteins
Skin the protective covering of the body often accounts for about 10 of the total body protein
2 Skin the protective covering of the body often accounts for about 10 of the total body protein
3 Many proteins are susceptible to alteration by a number of rather subtle changes in the immediate environment
4 A considerable amount of information is already available although much of it has been collected by biochemists using a specific food component as a model system
5 Several properties of amino acids provide evidence for this structure
6 The amino is responsible for the positive charge of lysine while arginine has a positively charged quanidino group
English Knowledge point
1 be of +名词相形容词
F： Proteins are molecules of great size complexity and diversity proteins are molecules of great size complexity and diversity roteins are molecules of great size complexity and diversity proteins are molecules of great size complexity and diversity
2 at PH 7 at neutral PH (介词at)
专业英语难点
SOME GOOD SENTENCE
1 Amino acids are the building blocks of proteins
Skin the protective covering of the body often accounts for about 10 of the total body protein
2 Skin the protective covering of the body often accounts for about 10 of the total body protein
3 Many proteins are susceptible to alteration by a number of rather subtle changes in the immediate environment
4 A considerable amount of information is already available although much of it has been collected by biochemists using a specific food component as a model system
5 Several properties of amino acids provide evidence for this structure
6 The amino is responsible for the positive charge of lysine while arginine has a positively charged quanidino group
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Proteins are molecules of great size complexity and diversity They are the source of dietary amino acids both essential and nonessential that are used for growth maintenance and the general wellbeing of man These macromolecules characterized by their nitrogen contents are involved in many vital processes intricately associated with all living matter In mammals and many internal organs are largely composed of proteins Mineral matter of bone is held together by collagenous protein Skin the protective covering of the body often accounts for about 10 of the total body protein
Some protein function as biocatalysts (enzymes and hormones) to regulate chemical reactions within the body Fundamental life process such as growth digestion and metabolism excretion conversion of chemical energy into mechanical work etc are controlled by enzymes and hormones Blood plasma proteins and hemoglobin regulate the osmotic pressure and PH of certain body fluids Proteins are necessary for immunology reactions Antibodies modified plasma globulin proteins defend against the invasion of foreign substances of microorganisms that can cause various diseases food allergies result when certain ingested proteins cause an apparent modification in the defense mechanism This leads to a variety of painful and occasionally drastic conditions in certain individuals
Food shortages exist in many areas of the world and they are likely to
become more acute and widespread as the world’s population increases providing
adequate supplies of protein poses a much greater problem than providing
adequate supplies of either carbohydrate or fat Proteins not only are more
costly to produce than fats or carbohydrates but the daily protein requirement
per kilogram of bodyweight remains constant throughout adult life whereas the
requirements for fats and carbohydrates generally decrease with age
As briefly described above proteins have diverse biological functions structures and properties Many proteins are susceptible to alteration by a number of rather subtle changes in the immediate environment Maximum knowledge of the composition structure and chemical properties of the raw materials especially proteins is required if contemporary and future processing of foods is to best meet the needs of mankind A considerable amount of information is already available although much of it has been collected by biochemists using a specific food component as a model system
Amino Acids
Amino acids are the building blocks of proteins Therefore to understand the properties of proteins a discussion of the structures and properties o f amino acids is required Amino acids are chemical compounds which contain both basic amino groups and acidic carboxyl groups Amino acids found in proteins have both the amino and carboxyl groups on the acarbon atom aamino acids have the following general structure
At neutral pH values in aqueous solutions both the amino and the carboxyl groups are ionized The carboxyl group loses a proton and obtains a negative charge while the amino group gains a proton and hence acquires a positive charge As a consequence amino acids possess dipolar characteristics The dipolar or zwitterions form of amino acids has the following general structure
Several properties of amino acids provide evidence for this structure they are more soluble in water than in less polar solvents when present in crystalline form they melt or decompose at relatively high temperatures (generally above 200) and they exhibit large dipole moments and large dielectric constants in neural aqueous solutions
The R groups or side chains of amino acids and proteins these side chains may be classified in to four groups
Amino acids with polaruncharged (hydrophilic) r groups can hydrogenbond with water and are generally soluble in aqueous solutions The hydroxyls of serine heroine and tyrosine the sulfhydryl of thinly of cysteine and the amides of asparagines and glutamine are the functional moieties present in r groups of the class of amino acids Two of these the toil of cysteine and the hydroxyl of tyrosine are slightly ionized at PG 7 and can lose a proton much more readily than others in this class The amides of asparagines and glutamine are readily hydrolyzed by acid or base to aspartic and glutamic acids respectively
Amino acids with nonpolar (hydrophobic) r groups are less soluble in aqueous solvents than amino acids with polar uncharged r groups Five amino acids with hydrocarbon side chains decrease in polarity as the length of the side chain is increased The unique structure of praline (and its hydoxylated derivative hydroxyproline) causes this amino acid to play a unique role in protein structure
The amino acids with positively charged (basic) r groups at ph 67 are lysine argiine has a positively charged quanidino group At ph 70 10 of the imidazole groups of histidine molecules are prorogated but more than 50 carry positive at ph 60
The dicarboxylic amino acids asparic glutamic possess net negative charges n the neutral ph range An important artificial mealflavoring food additive is the monosodium salt of glutamic acid
Peptides
When the amino group of one amino acid reacts with the carboxyl group of another amino acid a peptide bond is formed and a molecule of water is released This can bond joins amino acids together to form proteins
The peptide bond is slightly shorter than otter single cn bonds This indicates that the peptide bond has some characteristics of a double bond because of resonance stabilization with the carbony1 oxygen Thus group adjacent to the peptide bond cannot rotate freely this rigidity of the peptide bond holds
the six atoms in a single plane the amino (_NH_) group does not ionize between ph o and 14 due to the doublebond properties of the peptide bond In addition r groups on amino acid residues because of starch hindrance force oxygen and hydrogen of the peptide bond to exist on a trans configuration Therefore the backbone of peptides and proteins has free rotation in two of the three bonds between amino acids
If a few amino acids are joined together by peptide bonds the compound is called a most natural peptides are formed by the partial hydrolytic of proteins however a few peptides are important metabolites Ansetime and carnosine are two derivatives of histamine that are found in muscles pf animals The biochemical function of these peptides is not understood
Glutathione occurs in mammalian blood yeast and especially in tissues of rapidly dividing cells It is thought to function in oxidative metabolism and detoxification
Duirng oxidation two moletcules of glutathiune join vin a disulfide bridge (SS) between two cysteine is not found in proteins
Other peptides functino as antibodies and hormones Oxytocin and hormones Oxytocin and vasopressin are examples of peptide hormones
Protein structure
Proteins perform a wide variety of biological functions and since they are composed of hundreds of amino acids their structures are much mere complex than those of peptides
Enzymes are globular proteins produced in living matter for the special purpose of catalyzing vital chemical reactions that otherwise do not occur under physiological conditions Hemoglobin and myoglobin are hemocontaining proteins that transport oxygen and carbon dioxide in the blood and muscles The major muscle proteins actin and myosin convert chemical energy to mechanical work while proteins in tendons (collagen and elastim) bind muscles to bones skin hairy fingernails and toenails are pertinacious protective substance The food scientist is concerned about proteins in foods since knowledge of protein structure and behavior allows him to more ably manipulate foods for the benefit mankind
Nearly an infinite number of proteins could be synthesized from the 21natural occurring amino acids However it has been estimated that only about 2000 different proteins exist in nature The number is greater than this if one considers the slight variations found in proteins from different species
The linear sequence of amino acids in protein is referred toast primary structure In a few proteins the primary structure has been determined and one protein (ribonuclease) has been synthesized in the laboratory It is the unique sequence of amino acids that imparts many of the fundamental properties to different protein and tertiary structures If the protein contains a considerable number of amino acids with hydrophobic groups its solubility in aqueous solvents is probable less than that of proteins containing amino acids with many hydrophilic groups
If the primary structure of the protein were not folded protein molecules would be excessively long and thin A protein having a molecular weight of 13000 would be 448 a thick This structure allows excessive interaction with other substances and it is not found in nature The threedimensional manner in which relatively close members of the protein chain are arranged is referred to as secondary structure
examples or secondary structure are the ahelix of wool the pleatedsheet configuration of silk and the collagen helix
The native structure of a protein is that structure which possesses the lowest feasible free energy Therefore the structure of a protein is not random but somewhat ordered when the restrictions of the peptide bond are superimposed on a polyamino acid chain of a globular protein a right handed coil the ∝helix appears to be one of the most ordered and stable structures feasible
the ∝helix contains 36 amino acid residues per turn lof the protein backbone with the r groups of the amino acids extending outward from the axis of the helical structure hydrogen bonding can occur between the nitrogen of one peptide bond and the oxygen of another peptide bond four residues along the protein chain the hydrogen bonds are nearly parallel to the axis of the helix lending strength to the helical structure since this arrangement allows each peptide bond to form a hydrogen bond the stability of the structure greatly enhanced The coil of the helix is sufficiently compact and stables that even substances with strong tendencies to participate in hydrogen bonding such as water cannot enter the core
A secondary saturation found in many fibrous proteins is the βpleated sheet configuration In this configuration the peptide backbone forms a zigzag pattern with the r groups of the amino acids extending alive and below the peptide chain Since all peptide bonds are available for hydrogen bonding this configuration allows maximum crosslinking between adjacent polypeptide chains and thus good stability Both parallelpleated sheet where the polypeptide chains run in opposite directions are possible Where groups are bulky or have little charges the interactions of the r groups do not allow the pleatedsheet configuration to exist silk and insect fibers are the best examples of theβsheet although feathers of birds contain a complicated form of these configuration
Another type of secondary structure of fibrous proteins is the collagen helix collagen is the most abundant protein in higher vertebrates accounting for onethird of the total body protein collagen resists stretching is the major component of tendons and contains onethird glycine and onefourth proline or hydroxyprolinethe rigid r groups and the lack of hydrogen bonding by peptide linkages involving proline and hydroxyproline prevents formation of an ∝helical structure and forces the collagen polypeptide chain into an odd kinkedtype helix Peptide bonds composed of glycine form interchain hydrogen bonds with two other collagen polypeptide chains and this results in a stable triple helix This triplehelical structure is called tropocollagen and it has a molecular weight of 3000000 Daltons
The manner in which large portions of it protein chain are arranged is referred to as tertiary structure This involves folding of regular unts of the secondary structure as well as the structuring of areas of the peptide chain that are devoid of secondary structure for example some proteins contain areas where ∝helical structure exists and other areas where this structure cannot form depending on the amino acid sequence the length of the ∝helical portions are held together by hydrogen bonds formed between r groups by salt linkages by hydrophobic interactions and by covalent disulfide(ss0 linkages
The structures discussed so far have involved only a single peptide chain The structure formed when individual (subunit) polypeptide chains interact to form a native protein molecule is referred to as quaternary structure The bonding mechanisms that hold protein chains together are generally the same as those involved in tertiary structure with the possible exception that disulfide bonds do not assist in maitaining the quaternary structures of proteins
第四课 氨基酸蛋白质
蛋白质错综复杂种样分子物质食物必须氨基酸非必须氨基酸源体利氨基酸满足生长发育修复组织维持正常健康生活求分子含氮特征参许种生命物质复杂联系生命程包括类哺乳动物中蛋白质起着机体改造成分作肌肉许体器官蛋白质构成骨骼中矿物质胶原蛋白保持起机体保护层—皮肤中蛋白质通常占机体蛋白质总量10左右
蛋白质生物催化剂（酶激素）作调节体化学反应基生命程生长消化代谢排泄化学转变成机械功等等受酶激素控制某体液渗透压pH值受制血浆蛋白血红蛋白蛋白质免疫反应必少抗体（改性血浆球蛋白引起疾病外杂质微生物入侵某摄入蛋白质防御机制产生明显变化时便发生体生物敏导致某体身出现种样疾病时急剧病情
食物短缺现象世界许区存着口增加问题愈愈尖锐愈普遍蛋白质供应足问题远碳水化合物脂肪供应足更严重蛋白质仅产出费碳水化合物脂肪产出费高千克天需蛋白质量造整成年期恒定天需脂肪碳水化合物量般着年龄增长逐渐减少
正面简述样蛋白质种结构性质生理功许蛋白质容易受周围环境系列微妙变化影响发生变化想现食品加工理想满足类需必须彻底解原料特蛋白质组成结构化学性质目前已方面量资料供利中部分生物化学家利某特定食物成分作模拟物系加收集
氨基酸
氨基酸蛋白质结构单元解蛋白质性质旧需讨氨基酸结构性质氨基酸含氨基含酸性羧基化合物蛋白质中氨基酸α碳原子时氨基羧基α氨基酸具般结构：
中性pH水溶液中氨基羧基呈离子状态羧基失质子带负电荷时氨基质子带正电荷结果氨基酸便具偶极特性氨基酸种偶极形式（两性形式）般结构：
氨基酸种性质反映种结构性质：易溶水易溶极性溶剂：晶体形式存时较高温度（般200℃）熔化分解中性溶液种显示出偶极矩介电常数
氨基酸侧链R基团氨基酸蛋白质化学性质产生重影响侧链分四类
带极性非荷电（亲水）R基团氨基酸水形成氢键通常溶水溶液丝氨酸苏氨酸酪氨酸羟基半胱氨酸硫氢基（硫醇）天冬酰胺谷氨酰胺酰胺基时出现类氨基酸R基团中功部分中半胱氨酸硫羟基酪氨酸羟基pH7时轻度离子化类中氨基酸更容易失质子天冬酰胺谷氨酰胺酰胺基容易酸碱水解分形成天冬氨酸谷氨酸
带非极性（疏水）R集团氨基酸水溶液中溶解性带极性非荷电R基团氨基酸带烃侧链五种氨基酸侧链侧链长度增加降低脯氨酸（烃基衍生物羟脯氨酸）独特结构种氨基酸蛋白质结构中独特位
pH6～7时带正电荷（碱性）R基团氨基酸赖氨酸精氨酸组氨酸赖氨酸带正电原氨基精氨酸具带正电荷胍基pH7时组氨酸分子中咪唑基10质子化pH6时50带正电荷
二羟基氨基酸（天冬氨酸谷氨酸）中性pH范围带净负电荷谷氨酸钠盐种重膳食调味造食品添加剂
肽
氨基酸分子氨基氨基酸分子羧基起反应时便形成肽键时释放出分子水种CN键众氨基酸连接起形成蛋白质
种肽键简单CN键略短说明肽键羰基氧振稳定作肽键具定双键特性样紧邻肽键基团转动肽键种刚性六原子保持面 H
肽键双键性质亚氨基（NH)pH0～14间均离子化外立体位阻现象氨基酸残基R基团迫肽键氧原子氢原子反式构型存肽蛋白质链氨基酸间三键中两键作转动
果少数氨基酸肽键连接起样化合物称肽数天然肽蛋白质部分水解形成少数肽重代谢产物鹅肌肽肌肽动物肌肉中组氨酸两种衍生物肽生物化学功目前清楚
谷胱甘肽存哺乳动物血液酵母中特快速分解细胞质组织中般认种肽具参氧化代谢解毒作功氧化程中两分子谷胱甘肽通两半胱氨酸残基间二硫键SS连接起蛋白质中未发现谷氨酸γ羰基半胱氨酸连成肽键
外具抗体激素功肽催产素抗利尿素肽激素例子
蛋白质结构
蛋白质执行种样生物功数百氨基酸组成结构远肽复杂
酶生物中产生球状蛋白质目专门催化某生物化学反应然话化学反应生理条件会发生血红蛋白肌红蛋白输送血液肌肉中氧二氧化碳含血红素蛋白质重肌肉蛋白—肌动蛋白肌球蛋白化学转变成机械腱中蛋白质（胶原蛋白弹性蛋白）肌肉粘连骨骼皮肤毛发指（趾）甲蛋白质类保护物质食品科学家关注食物蛋白质掌握蛋白质结构功方面知识更加工处理食品造福类
21种天然存氨基酸合成数蛋白质估计然界存约2000种蛋白质果考虑物种蛋白质存微差异蛋白质数目超数
蛋白质分子氨基酸直线排列次序作蛋白质级结构少数蛋白质级结构已确定已实验室里合成中种蛋白质（核糖核酸）正种氨基酸独特排列序赋予蛋白质许基特性程度决定二级三级结构果蛋白质中含量带疏水基团氨基酸水溶剂中溶解性带许亲水基团氨基酸蛋白质差
果蛋白质级结构折叠蛋白质分子会长细分子量13000蛋白质分子应448纳米长37纳米粗种结构物质发生度相互反应然样结构然界未发现蛋白质链中相互链节链节间三维排列方式蛋白质二级结构二级结构具体例子羊毛蛋白α螺旋结构蚕丝蛋白折叠片结构胶原蛋白螺旋结构
蛋白质然结构含低结构蛋白质结构意定规球状蛋白质聚氨基酸链受肽键约束时右螺旋（α螺旋）规稳定合理结构
圈α螺旋蛋白质链含36氨基酸残基氨基酸R基团螺旋结构轴线外伸出肽键氮够蛋白质链相距四氨基酸残基处氧形成氢键氢键差α螺旋轴线行赋予螺旋结构强度样排列肽键形成氢键加强结构稳定性螺旋圈非常紧密坚固象水样强烈参形成氢键趋势物质进入螺旋中央部分
出现许纤维状蛋白质中二级结构β折叠片结构构型中肽链呈锯齿形氨基酸R基团肽链方方伸展肽键供氢键形成种构型够相邻肽链间充分形成交联具良稳定性两种折叠片相邻肽链走相行折叠片走相反反行折叠片均存果肽链中R基团带种电荷R基团间相互作β折叠片形成蚕丝昆虫纤维蛋白β折叠结构例子鸟类羽毛中含种构型复杂形式
纤维蛋白种二极结构胶原螺旋胶原螺旋高等脊椎动物中丰富种蛋白质占动物体蛋白总量13胶原蛋白中含1＼3甘氨酸1＼4脯氨酸羟脯氨酸抵抗拉伸腱组分R基团刚性脯氨酸羟脯氨酸参肽式键合形成氢键原α螺旋结构法形成迫胶原肽链变成种零散结节式螺旋体胶原肽链中甘氨酸构成肽键两条肽链形成键间氢键产生种稳定三股螺旋三股螺旋结构称原胶原分子量30万道尔顿
蛋白质链中链段排列方式称蛋白质三级结构包括二级结构常规单元折叠二级结构肽链干区域结构化例某蛋白质中包含α螺旋结构存区域外形成种结构区域根氨基酸序α螺旋段长度赋予独特三级结构折叠部分R基团间形成氢键盐键疏水相互作价二硫键（SS）结合起
讨结构仅涉单肽链结构（亚单位）肽链相互作变成天然蛋白质分子时形成结构蛋白质四级结构蛋白质键结合起键合机制通常三级结构中述相例外情况双硫键参蛋白质四级结构保持
专业英语词汇
intricate a复杂错综缠结难懂
collagenous a胶原
globulin 球蛋白
plasma 血浆原生质
immunological 免疫
hemoglobin 血红蛋白
basic amino 碱性氨基
acidic carboxyl 酸性羧基
aqueous ①水 ②含水 ③水成
proton 质子氕核
dipolar 偶极两极
zwitterion 两性离子
crystalline ①结晶晶状 ②清澈
hydrophilic 亲水
serine 丝氨酸羟基丙氨酸
threonine 羟基丁氨酸苏氨酸
tyrosine 酪氨酸 3羟苯基丙氨酸
sulfhydryl 氢硫 ～enzyme 硫化氢解酶 ～ group 巯基
cysteine 半胱氨酸巯基丙氨酸
cystine 胱氨酸双巯丙氨酸
amide ①酰胺 ②氨化物
asparagine 天门冬酰胺
glutamine 谷氨酰胺
aspartic acid 天门冬氨酸丁氨二酸
glutamic acid 谷氨酸
proline 脯氨酸氮戊环[2]基羧酸
lysine 赖氨酸
arginine 精氨酸
histidine 组氨酸咪唑丙氨酸
quanidino 胍
imidazol n 咪唑13二氮杂茂
resonance n ①回声反响 ②[物]振鸣谐振 ③[医]叩响
imino 亚氨
steric a空间位
anserine 鹅肌肽
carnosine 肌肽
glutathione 谷胱甘肽
peptide肽缩氨酸
oxytocin n(垂体)叶催产素
vasopressin n叶加(血)压素加压素
carbonyl 羰基碳酰
hemoglobin 血红蛋白 hemo表示血
myoglobin 肌红蛋白
actin 肌动蛋白
myosin 肌球蛋白
tendon 腱筋根
collagen 胶原胶原蛋白
elastin 弹性蛋白
ribonuclease 核糖核酸酶
hydrophobic 疏水
restriction n 限制限定约束
vertebrate n脊椎动物 a①椎骨脊椎脊椎动物 ②(作品等)结构严密
kink n①(绳索头发)细结绞缠 ②奇想怪念头乖僻 ③(奇特)妙法 ④(颈背等处)
肌肉痉挛抽筋 ⑤[美](结构设计等)缺陷 vt纽结绞缠 vi纽结结
glycine 甘氨酸氨基醋酸
tropocollagen 原胶原
dalton 道尔顿
devoid a 缺乏没(of)
covalent 价 ～ bond价键
quaternary a①四组四部组成第四 ②四元四价季 ③[]第四纪
n①四四组第四组中组成部分 ②[数]四进制 ③[]第四纪
zigzag ①字形Z字形锯齿形②字形线条（道路壕沟装饰等）③蜿蜒曲折盘旋弯曲
专业英语总结
English Knowledge point
1 be of +名词相形容词
F： Proteins are molecules of great size complexity and diversity proteins are molecules of great size complexity and diversity roteins are molecules of great size complexity and diversity proteins are molecules of great size complexity and diversity
2 at PH 7 at neutral PH (介词at)
SOME GOOD SENTENCE
1 Amino acids are the building blocks of proteins
Skin the protective covering of the body often accounts for about 10 of the total body protein
2 Skin the protective covering of the body often accounts for about 10 of the total body protein
3 Many proteins are susceptible to alteration by a number of rather subtle changes in the immediate environment
4 A considerable amount of information is already available although much of it has been collected by biochemists using a specific food component as a model system
5 Several properties of amino acids provide evidence for this structure
6 The amino is responsible for the positive charge of lysine while arginine has a positively charged quanidino group
English Knowledge point
1 be of +名词相形容词
F： Proteins are molecules of great size complexity and diversity proteins are molecules of great size complexity and diversity roteins are molecules of great size complexity and diversity proteins are molecules of great size complexity and diversity
2 at PH 7 at neutral PH (介词at)
专业英语难点
SOME GOOD SENTENCE
1 Amino acids are the building blocks of proteins
Skin the protective covering of the body often accounts for about 10 of the total body protein
2 Skin the protective covering of the body often accounts for about 10 of the total body protein
3 Many proteins are susceptible to alteration by a number of rather subtle changes in the immediate environment
4 A considerable amount of information is already available although much of it has been collected by biochemists using a specific food component as a model system
5 Several properties of amino acids provide evidence for this structure
6 The amino is responsible for the positive charge of lysine while arginine has a positively charged quanidino group
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